One of the life limiting factors for many heat recovery steam generators (HRSGs) is associated with damage that occurs at header-to-tube attachments. This attachment is particularly troublesome due to thermal differences experienced between the header and the tubes during cyclic operation. Tubes attached to the header tend to cool very rapidly to the temperature of the incoming water and the bulk wall temperature of the header tends to respond much more slowly. Thermal shock results and often leads to thermal fatigue failures at the toe of the tube weld. Corrosion fatigue in the vicinity of the root of the toe welds have also been noted for units that experience extensive cycling.
Restricted axial expansion of tubes induces forces and bending moments on both tubes and the tube attachments. Tubes are restricted from moving as they are attached to inlet and outlet headers at each end. This results in restricted thermal expansion. Tube temperatures quickly follow the temperature of the fluid flowing through them. Anomalies in temperature and flow rate of the fluid between tubes in the same row can cause significant differences in the amperage temperatures of the set of tubes attached to the same headers. During start-ups and shutdowns, these transient conditions place abnormal stresses on the tubes and tube attachments, eventually resulting in damage to the attachments.
At present, the industry addresses tube attachment damage from the outside of the header. This requires cutting your way past a number of tubes in a tube bundle, performing the weld repair from the outside of the tube (often requiring partial tube replacement), and then re-welding all of the tubes that were cut to gain access to the header-to-tube damage. This approach (commonly referred to as “cutting your way in and welding your way out”) is time consuming, costly, and more often than not, results in poor weld quality due to limited accessibility.
Commonly assigned U.S. Pat. No. 6,596,957 is directed to a method and apparatus and prefabricated replacement tube for localized waterwall repair, and is incorporated by reference as if fully written out below. Also, commonly assigned U.S. Patent Application entitled “Method for Inspection and Repair”, filed on even date, is incorporated by reference as if fully written out below.